missing HexString documentation

[Walter Bright]

On 2/7/2018 6:39 PM, H. S. Teoh wrote:
>> and hope to make a PR forit shortly.
>>
>>    https://issues.dlang.org/show_bug.cgi?id=18397
> 
> The bug report didn't explain what exactly in the implementation wasn't
> done right. :-/

The PR does.

https://github.com/dlang/phobos/pull/6138


> Another data point: instantiating 10000 hex literals causes compilation
> time to bloat to 10 seconds.  While I'm not saying we should expect user
> code to have so many hex literals, the point is that that's unacceptably
> slow for D, given our motto of fast-this and fast-that.

Try it with the new PR.